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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

A digitized atomic clock based on transient oscillation of detuned coherent population trapping

Zhong Wang, Jianye Zhao, Wan Mingyu, Yeqing Li, Daiting Shi, Dong Hou, Shuangyou Zhang, Dawei Li

Published Jul 1, 2013• 1 citation
Publisher

Abstract

We demonstrated a digitized atomic clock which is based on the transient oscillation of detuned coherent population trapping (DCPT). In the Λ structure atomic states system, the DCPT oscillation frequency exactly equals the absolute value of the frequency difference between the two pump laser fields minus the two ground states hyperfine splitting frequency. This DCPT atomic clock obtains the standard frequency by adding/subtracting the DCPT frequency from the laser modulation RF frequency. And it does not require a phase locking loop circuit to lock the standard frequency to the atomic transition. In addition, its data processing section can be easily digitized with high accuracy. The frequency instability of 1×10-11/τ1/2(τ=1000s) has been achieved.

DOI
10.1109/eftf-ifc.2013.6702060

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We demonstrated a digitized atomic clock which is based on the transient oscillation of detuned coherent population trapping (DCPT). In the Λ structure atomic states system, the DCPT oscillation frequency exactly equals the absolute value of the frequency difference between the two pump laser fields minus the two ground states hyperfine splitting frequency. This DCPT atomic clock obtains the standard frequency by adding/subtracting the DCPT frequency from the laser modulation RF frequency. And it does not require a phase locking loop circuit to lock the standard frequency to the atomic transition. In addition, its data processing section can be easily digitized with high accuracy. The frequency instability of 1×10-11/τ1/2(τ=1000s) has been achieved.
We proposes a method for measuring the transient oscillation frequency observed when Coherent Population Trapping is excited by means of optical pumping radiation fields whose frequency difference is detuned slightly from the ground state hyperfine frequency of the atom used. Using narrow-band simulation filter and phase matching techniques this measuring method can provides a measurement precision of about 0.5Hz. Based on this reading accuracy the DCPT atomic clocks frequency instability can be better than 5e-11 (t=100s) and tend to more stable at longer integration time. According to our experimental results, we demonstrate that a DCPT atomic frequency standard could give good short and long term frequency stability.
We proposed a scheme to implement coherent population trapping (CPT) atomic clock based on the transient CPT phenomenon. We proved that the transient transmitted laser power in a typical Λ system near CPT resonance features as a damping oscillation. Also, the oscillating frequency is exactly equal to the frequency detuning from the atomic hyperfine splitting. Therefore, we can directly measure the frequency detuning and then compensated to the output frequency of microwave oscillator to get the standard frequency. By this method, we can further simplify the structure of CPT atomic clock, and make it easier to be digitized and miniaturized.
We have theoretically and experimentally proven that the transient transmitted laser power through a typical Λ system atomic cell near Coherent Population Trapping resonance presents as a damping oscillation. The oscillating frequency is identical to the frequency detuning from the atomic hyperfine splitting. Based on the transient coherent population trapping (TCPT) phenomenon, we have proposed an atomic clock in which the standard frequency is obtained by compensated the TCPT oscillating frequency to the RF modulating frequency. Therefore we do not need to lock the RF frequency. There will be advantages for atomic clock miniaturization and stabilization.
We demonstrated an atomic clock based on coherent population beating (CPB). CPB occurs in typical Λ system, which enables us to obtain the beat frequency of the radio frequency for laser modulation and the hyperfine splitting. The beat frequency and its fluctuation is detected and accurately measured through digitized processing for compensating the shift of the radio frequency. It enables us to achieve a standard frequency which directly reference to the hyperfine transition. In our preliminary experiment an Allan Deviation of 3e-12 at 1000s is observed. CPB atomic clock eliminates the phase locking loop, and can be easily digitized and integrated.
We demonstrated a frequency standard based on a detuned coherent population beating phenomenon. In this phenomenon, the beat frequency of the radio frequency for laser modulation and the hyperfine splitting can be obtained by digital signal processing technology. After analyzing the spectrum of the beat frequency, the fluctuation information is obtained and applied to compensate for the frequency shift to generate the standard frequency by the digital synthesis method. Frequency instability of 2.6 × 10 12 at 1000 s is observed in our preliminary experiment. By eliminating the phase-locking loop, the method will enable us to achieve a full-digital frequency standard with remarkable stability.
Space-borne atomic frequency standards demand a high level of performance and stability throughout the mission life in the harsh environment of space. Recently, atomic clocks employing Coherent Population Trapping (CPT), a quantum interference phenomenon in alkali atoms, have shown much promise for space application. This paper presents the development and realization of compact and reliable CPT atomic clock for ISRO's navigational satellites. In our approach, quantum interference in Rb-87 atoms are pursued for obtaining the clock resonance, owing to their larger ground state hyperfine transition energy (higher Q-factor). Quantum interference is realized by generating two resonant optical laser fields from a VCSEL diode by modulating its drive current at RF frequency which corresponds to the ground state hyperfine separation in Rb-87 atoms. The emission frequency of the laser diode is stabilized by locking it with respect to the D1 hyperfine transition with the help of a servo control loop, thereby the instabilities associated with laser frequency fluctuations are minimized. The resulting CPT resonance (<;kHz) as shown in Figure 1 is used as reference to stabilize the crystal oscillator frequency (10 MHz) that serves both as clock output and seed for generating the RF field.